Hi there,
As for the backpack, I would recommend you stick with your 38L one, if you get the 60L one you will it with all kinds of stuff you won't need. 38L will be more than enought to get you throught he WHW if you are not planning on camping. Going light is the way to go when it comes to long distance. A good rule is, if you don't have it, you don't need it! It is very tempting to take stuff you think you might possibly need, but never do, adding up to too much weight. By limiting the size of your bag, you make sure you only take what you really need. And the Kestrel is an excellent bag!
For reference, I walked 3.5 months with camping with the 48L kestrel!
(earlier I tried and failed on the WHW, simply because I took too much crap)
Waterproofs, very useful! My personal preference is the Berghaus Paclite kit (overtrousers and jacket), super light, stronger than it looks, very waterproof.
Trousers, one pair is plenty. Proper hiking trousers, like your fjallraven, will dry very quickly if it gets wet, so that is all you need really. Don't worry about it getting it a bit dirty or smelly, that is expected on a long distance walk, all places along the way will be perfectly fine with that.
2 fleeces is also too much, I would recommend two good base layers, quick drying and wicking, one midlayer (fleece or ideally polartec microfleece or the like as it keeps you just as warm and weighs less). That is pretty much it, the baselayers are quick drying, so every second day I would rinse one out in the b&b sink and let it dry, attached to your backpack, as you walk, then just keep swapping them out. You won't need to worry about getting cold, if you do get cold, just pop on the goretex jacket, it will trap the heat in and you will be fine. I have never been cold with that combination even on winter walks in -10 and more.... as long as you keep moving, but since it won't be that cold when you do the trip, you will be perfectly fine.
As for underwear, if you get proper hiking underwear, quick drying, wicking, anti-bacterial etc, you can get away with wearing one pair for 2-3 days easily, so same deal here, bring two, wear one, wash one, keep swapping.
I use Craft underwear, brilliant stuff! no chaffing, no discomfort!
Socks I would take 3 pairs as I tend to swap those more often, but that is mainly due to my feet being useless and blister prone!
Hiking poles are very useful in my opinion. I would recommend stashing them away for the uphill sections (good exercise for your legs and strengthening the knees), but use them for any downhill walking, that stuff is no good for knees! Also handy when you start to get tired, it is very easy to stop paying attention after a while and do your ankle in, poles can be great for quickly takign your weight off your feet if that does happen!
(attach a pole on each side of your pack and you can hang your clothes across them to dry!)
I would highly recommend drybags, first to keep your stuff dry of course, but also to keep order and allow you to quickly find your kit without having to take everything out. (get different coloured ones). also great for looking for something in your bag while it rains without getting everything wet.
I have never used a midgy net, as long as you keep moving, those little bastards won't bother you, but probably not a bad idea to take one, I always planned on trying one, so cannot give you any advice there really.
Other stuff I would always take:
- compass
- gaiters
- small sealable plastic bag for money/credit card (wallet weighs too much and takes up space)
- some paracord (attaching stuff to your pack, emergency shoe lace replacement etc)
- swiss army knife
- headtorch plus spare batteries
- kindle (evening entertainment)
- basic first aid kit (super glue, duct tape, ibuprofene gel, paracematol, blister plasters)
- if you have the space, some light shoes to wear in the evening, always a relief to get those boots off. the lighter the better. vibram five fingers are pretty good if you don't mind getting weird looks from people!
That is pretty much all you will need!
Sorry to hear your friend had to drop out on you, form my experience though, that may not necessarily be a bad thing for you, long distance walks on your own and a fantastic experience, you will be able to walk your own pace, are more likely to get in contact with other random people walking the way and it will give you lots of time to contemplate life
Have fun and good luck